System and method for fabricating diodes

Abstract

This invention is directed to a system and method of fabricating PN and PiN diodes by diffusing an acceptor impurity into a substrate. This invention is particularly advantageous for fabricating SiC diodes having linearly graded, deep pn junctions. One method that this invention uses to achieve its advantages is by diffusing an acceptor impurity into a substrate using a crucible, acceptor source, substrate, and furnace.

1 . A method for fabricating a diode comprising the steps of: 
 providing a crucible;    providing a substrate comprised of wide band gap material having at least one epitaxial layer within said crucible;    providing an acceptor impurity in a gaseous state within said crucible; and,    heating said crucible to at least 1400° C. for at least 60 seconds so that said acceptor diffuses into said substrate to create a p layer thereby producing a PiN diode.    
   
   
       2 . The method of  claim 1  wherein said step of providing a substrate includes the step of providing a substrate doped by donor impurities with a density of at least 10 17  cm −3 .  
   
   
       3 . The method of  claim 1  wherein said step of providing a substrate includes the step of providing a substrate doped with an n-type epitaxial layer with a density less than 10 17  cm −3 .  
   
   
       4 . The method of  claim 1  wherein said step of providing a substrate includes the step of providing a SiC substrate.  
   
   
       5 . The method of  claim 1  including the steps of: 
 forming a photoresistant layer on said substrate; and,    heating said photoresistant layer in a vacuum to at least 400° C. at a rate of 2000° C. per hour or less so that said heating forms a carbon film thereby providing a protection layer on said substrate.    
   
   
       6 . The method of  claim 5  wherein the step of heating said photoresist layer in a vacuum of pressure equal or below 1 Torr.  
   
   
       7 . The method of  claim 1  wherein the step of providing an acceptor impurity in a gaseous state includes the step of providing source material having Boron.  
   
   
       8 . The method of  claim 7  wherein the step of providing source material includes the step of providing SiC powder having at least 0.01% Boron by weight.  
   
   
       9 . The method of  claim 1  including the step of forming mesa structures on said substrate so that a PiN diode is formed.  
   
   
       10 . The method of  claim 1  including the step of forming a local mask on said substrate so that local diffusion can be performed.  
   
   
       11 . The method of  claim 10  wherein the step of forming a local mask includes the steps of: 
 forming a local photoresist pattern having a thickness of more than 2 microns;    heating said photoresist pattern to at least 200° C. at a rate of 400° C. per hour or less to provide a hard-baked photoresist; and,    heating said hard-baked photoresist in a vacuum at a pressure equal to or below 1 Torr to at least 600° C. at a rate of 400° C. per hour or less.    
   
   
       12 . The method of  claim 1  including the steps of: 
 evacuating said crucible to a pressure of 760 Torr or less; and,    inserting an inert gas into said crucible prior to heating said crucible to at least 1400° C.    
   
   
       13 . A method for fabricating a diode comprising the steps of: 
 providing a crucible;    providing a substrate comprised of wide band gap material having a n +  layer, a n −  layer and a p +  layer within said crucible;    providing an acceptor impurity in a gaseous state within said crucible; and,    heating said crucible to at least 1400° C. for at least 60 seconds so that said Boron diffuses into said substrate to create a p-layer thereby fabricating a PiN diode.    
   
   
       14 . The method of  claim 13  wherein said step of providing a substrate includes the step of providing a substrate doped by donor impurities with a density of at least 10 17  cm −3 .  
   
   
       15 . The method of  claim 13  wherein said step of providing a substrate includes the step of providing a substrate with an n-type epitaxial layer with a density less than  10   17  cm −3 .  
   
   
       16 . The method of  claim 13  wherein said step of providing a substrate includes the step of providing a substrate with a p-type layer.  
   
   
       17 . The method of  claim 13  including the steps of: 
 forming a photoresist layer on said substrate; and,    heating said photoresist layer in a vacuum to at least 400° C. at a rate of 2000° C. per hour or less so that said heating forms a carbon film thereby providing a protection layer on said substrate.    
   
   
       18 . The method of  claim 13  wherein the step of providing an acceptor impurity in a gaseous state includes the steps of: 
 providing source material having Boron; and,    evacuating said crucible to a pressure below or equal to 1 Torr.    
   
   
       19 . The method of  claim 18  including the steps of: 
 evacuating said crucible to a pressure of 760 Torr or less; and,    inserting an inert gas into said crucible prior to heating said crucible to at least 1400° C.    
   
   
       20 . A system for fabricating diodes comprising: 
 a housing;    a crucible contained within said housing;    a heating member for heating said crucible;    a substrate contained within said crucible having donor impurities with a density of at least 10 17  cm −3  and a n-type epitaxial layer with a density of less than or equal to 10 17  cm −3 ; and,    a gaseous acceptor impurity source contained within said crucible so that when said crucible is heated, said acceptor diffuses into said substrate to form a PiN diode.    
   
   
       21 . The system of  claim 20  including a gaseous Aluminum source contained within said crucible so that said Aluminum diffuses into said substrate when heated.  
   
   
       22 . The system of  claim 21  including an inert gas contained within said crucible.  
   
   
       23 . The system of  claim 21  including insulation material generally surrounding said crucible.  
   
   
       24 . The system of  claim 20  wherein said gaseous acceptor impurity source includes SiC powder.